Solid electrolytic capacitor and process for producing the same

ABSTRACT

A solid electrolytic capacitor includes a valve metal on which surface a dielectric oxide film layer, a solid electrolyte layer, and a cathode layer are formed in this order. The cathode layer includes a silver layer which is formed of silver particles and at least one of phenolic novolak type epoxy resin represented by formula (1) and trishydroxyphenylmethane type epoxy resin represented by formula (2). Not less than 90 wt. % of the silver particles are occupied by flaky silver particles, which account for not less than 50 vol. % and not greater than 90 vol. %. This capacitor shows excellent characteristics in ESR and impedance.

This application is a U.S. national phase application of PCTinternational application PCT/JP2003/014350 filed Nov. 12, 2003.

TECHNICAL FIELD

The present invention relates to solid electrolytic capacitors and aprocess for producing the same capacitors.

BACKGROUND ART

Electronic devices have been digitized recently, and this market trenddemands capacitors used in those devices to have a low impedance at ahigh frequency region and a greater capacity in a downsized body. Inorder to meet this demand, plastic-film capacitors, mica-capacitors, orlaminated ceramic capacitors are used. Other than those capacitors,aluminum electrolytic capacitors, aluminum solid electrolyticcapacitors, tantalum solid electrolytic capacitors are also used formeeting the foregoing demand.

An aluminum solid electrolytic capacitor is formed by this method:positive and negative electrodes etched and made of aluminum foil arewound up with a separator therebetween, and a liquid electrolyte isused. An aluminum solid electrolytic capacitor and a tantalum solidelectrolytic capacitor aim to improve the capacitor properties at ahigh-frequency region. The electrolyte of those capacitors is made ofsolid electrolyte such as conductive polymer or manganese oxide; theconductive polymer is formed by polymerizing polymeric monomer such aspyrrole or thiophene derivatives. Those solid electrolytic capacitorshave been developed and are now available in the market.

FIGS. 6A and 6B show structure of a capacitor element to be used insolid electrolytic capacitors. FIG. 6A shows a perspective view of thecapacitor element and FIG. 6B shows a sectional view of the elementshown in FIG. 6A taken along line 6B—6B. Valve metal 31 is roughened byetching process, and has anodic oxide film 32 (hereinafter referred tosimply as “film”) on its surface. Insulating tape 33 disposed on film 32divides valve metal 31 into anode leader 31A and capacitor elementsection 31B. On the surface of film 32 of capacitor element section 31B,the following two layers are formed in this order: solid electrolytelayer 34 made of conductive polymer, and conductive layer 35 made of acarbon layer and a silver paste layer. Capacitor element 36 is thusconstructed.

Anode leader 31A and conductive layer 35 are coupled to an anodeterminal and a cathode terminal respectively (not shown). The wholecapacitor element 36 is covered by an outer casing resin (not shown)formed by molding, so that a solid electrolytic capacitor is obtained.

An electrolytic oxidation polymerization method and a chemical oxidationpolymerization method are known as methods of forming solid electrolytelayer 34. According to the former method, a manganese dioxide layer isformed in advance on film 32, and solid electrolyte layer 34 is formedon the manganese dioxide layer. According to the latter method, solidelectrolyte layer 34 is formed on film 32 directly.

The carbon layer and the silver paste layer are formed by applying therespective pastes available in the market and drying them.

The prior art related to the present invention is Japanese PatentApplication Unexamined Publication No. H05-159987.

The characteristics of the foregoing solid electrolytic capacitorlargely depend on conductive layer 35 made of the carbon layer and thesilver paste layer formed on the surface of solid electrolyte layer 34.In particular, material of silver particles of the silver paste layer,its particle shape, a ratio of resin material vs. silver particlesaffect an equivalent series resistance (hereinafter referred to as“ESR”) of the capacitor characteristics.

However, optimization of the silver particles material and its particleshape as well as the ratio of the silver particles vs. epoxy resin(reactant of bisphenol A and epichlorohydrin) available in the marketcannot achieve a capacitor that satisfies the characteristics in a highfrequency region needed for the digitization of electronic devices.

An interface resistance between the carbon layer and the silver pastelayer becomes high depending on a surface condition of solid electrolytelayer 34, so that the ESR of the capacitor becomes high.

DISCLOSURE OF INVENTION

A solid electrolytic capacitor of the present invention has a valvemetal and, formed on a surface of the metal in this order: a dielectricoxide film layer, a solid electrolyte layer, and a cathode layer. A partof the cathode layer has a silver layer, which includes silver particlesand at least either of phenolic novolak type epoxy resin represented byformula (1) and trishydroxyphenylmethane type epoxy resin represented byformula (2). At least 90 wt. % of the silver particles are flaky, andthe flaky particles account for 50 to 90 vol. %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view illustrating a structure of a capacitorelement of a solid electrolytic capacitor in accordance with exemplaryembodiments of the present invention.

FIG. 2 shows characteristics illustrating a relation between a volumeoccupied by flaky silver particles included in a silver layer and ESRcharacteristics of the solid electrolytic capacitor in accordance with afirst exemplary embodiment of the present invention.

FIG. 3 shows characteristics illustrating a relation between a ratio offlaky silver particles vs. spherical particles and ESR characteristicsof the solid electrolytic capacitor in accordance with the firstexemplary embodiment of the present invention.

FIG. 4 shows characteristics illustrating a relation between a curingtemperature of a silver layer and ESR characteristics of the solidelectrolytic capacitor in accordance with a second exemplary embodimentof the present invention.

FIG. 5 shows characteristics illustrating a relation between a heattreatment temperature of silver paste and curing stress of the silverlayer in accordance with the second exemplary embodiment of the presentinvention.

FIG. 6A shows a perspective view illustrating a structure of a capacitorelement of a conventional solid electrolytic capacitor.

FIG. 6B shows a sectional view of the capacitor element shown in FIG.6A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are demonstratedhereinafter. Similar elements to those in the respective embodimentshave the same reference marks and detailed description thereof isomitted.

First Exemplary Embodiment

FIG. 1 shows a sectional view illustrating a structure of a capacitorelement of a solid electrolytic capacitor in accordance with the firstexemplary embodiment of the present invention. Aluminum foil 11undergoes a surface roughening process, and a dielectric oxide filmlayer 12 (hereinafter referred to simply as “film”) is formed thereon.Insulating layer 15 divides foil 11 into anode section 13 and cathodesection 14. In cathode section 14, solid electrolyte layer 16 andcathode layer 17 are formed in this order, thereby forming capacitorelement 18. Layer 16 can be a single layer or plural layers. Cathodelayer 17 is formed of carbon layer 19 and silver layer 20.

Anode section 13 and cathode section 17 are coupled to respectiveterminals (not shown) for outer devices. An outer casing resin (notshown) covers capacitor element 18 with the respective terminals exposedpartially. A solid electrolytic capacitor is thus obtained.

To be more specific, the solid electrolytic capacitor is constructed inthe following way:

In sample Group 1, first, a surface of foil 11 of 10 μm thickness isroughened by etching in an electric chemical way, then foil 11 havingundergone the surface roughening is dipped into water solution of 3%ammonium adipate. Then foil 11 undergoes anodic oxidation by beingapplied a voltage of 12V for 60 minutes at 70° C. in the water solution,so that film 12 is formed.

Next, foil 11 on which film 12 is formed is shaped into a belt-like formhaving a width of 6 mm, and insulating layer 15 made of polyimide resintape is stuck to foil 11 in order to divide the surface of foil 11 intoanode section 13 and cathode section 14.

Cathode section 14 is press-molded into a comb-like shape, and acomb-like shaped cross section of foil 11 undergoes a chemical treatmentsimilar to that used to electrode foils of aluminum electrolyticcapacitors.

Next, cathode section 14 is dipped into water solution of 30% manganesenitrate, and undergoes air-drying before it undergoes heat decompositionat 300° C. for 10 minutes, so that a manganese oxide layer as aconductive layer (not shown) is formed. This layer is a part of thesolid electrolyte layer.

Polymerizing liquid for forming the solid electrolyte layer is preparednext. Pyrrole monomer 0.5 mol/L and sodium propylnaphthalene-sulfonate0.1 mol/L are mixed in advance. Water as solvent and propyl-phosphateester as pH adjuster are added to the foregoing mixture, so that pH ofthe mixture is adjusted to 2. Cathode section 14 is dipped into thispolymerizing liquid, and an electrode for starting polymerization isplaced close to the surface of cathode section 14, so that electrolyticoxidizing polymerization starts. Solid electrolyte layer 16 made ofconductive polymer is thus formed on the surface of the conductivelayer.

Then, Colloidal carbon suspension is applied and dried, so that carbonlayer 19 is formed, and silver paste is applied and dried, so thatsilver layer 20 is formed to be cathode layer 17. Silver layer 20 isformed by dipping into silver paste before heat treatment at 200° C. for10 minutes is provided.

Belt-like foil 11 is punched so that anode section 13 can be obtained,thereby producing capacitor element 18.

Capacitor elements 18 are laminated, and a terminal for an outer deviceis coupled to anode sections 13 of the laminated body. Another terminalis coupled to cathode layers 17, and outer casing resin, made of e.g.epoxy resin, covers laminated elements 18 with the terminals partiallyexposed, so that a solid electrolytic capacitor is obtained. Thiscapacitor is rated at 6.3V and 10 μF.

The silver paste forming of silver layer 20 has dispersion of particlediameters ranging from 0.1 μm to 30 μm (average diameter; 4 μm) andincludes flaky silver particles of which dimension, in terms of a ratioof vertical length vs. horizontal length, disperses from 1:2 to 1:10.This silver paste includes phenolic novolak type epoxy resin representedby formula (1), hydroquinone diglycidyl ether as curing agent, andethylene glycol monobutyl ether as diluting agent. When those materialsare mixed, a mixing ratio of silver particles vs. phenolic novolak typeepoxy resin is changed so that the silver particles can occupy thevolume at 40, 50, 60, 70, 80, 90, and 95%.

Next, sample Group 2 is described hereinafter. In sample group 2,trishydroxyphenylmethane type epoxy resin represented by formula (2) isused as material of the silver paste instead of phenolic novolak typeepoxy resin used in sample group 1. Other materials remain unchangedfrom those of sample group 1, and the volume occupation of the flakysilver particles is varied for producing solid electrolytic capacitorsas same as sample group 1.

Next, sample Group 3 is described hereinafter. In sample group 3,phenolic novolak type epoxy resin represented by formula (1) andtrishydroxyphenylmethane type epoxy resin represented by formula (2) areused at a compounding ratio of 50:50 as material of the silver pasteforming silver layer 20. Other materials remain unchanged from those insample group 1, and the volume occupation of the flaky silver particlesis varied for producing solid electrolytic capacitors as same as samplegroup 1.

The silver paste includes curing agent and diluting agent in addition tothe epoxy resins represented by formulae (1) and (2). Curing acceleratoror coupling agent can be used if necessary. As curing agent,hexahydrophthalic anhydride, methylhydrophthalic anhydride, polyphenolicmaterial such as phenolic novolak type epoxy resin, or amine-basedcompound such as imidazole, dicyandiamide, is used. As diluting agent,alcohol-based solvent, cellosolve-based solvent, carbitol-based solvent,ester-based solvent, or ketone-based solvent can be used.

FIG. 2 shows ESR characteristics measured at 100 kHz of the solidelectrolytic capacitors immediately after being produced according tothe methods of foregoing sample groups 1, 2 and 3. ESR characteristicsof a solid electrolytic capacitor of another sample group for comparisonpurpose are listed in FIG. 2. This capacitor uses bisphenol A type epoxyresin instead of phenolic novolak type epoxy resin used in sample group1.

As FIG. 2 tells, the solid electrolytic capacitors of sample groups 1–3,of which volume occupancy of the flaky silver particles in silver layerranges from 50 to 90%, are excellent in ESR characteristics, namely, notmore than 100 mΩ. On the other hand, capacitors of which volumeoccupancy of flaky particles is less than 50% or exceeds 90% are poor inESR characteristics. The reason is this: in the case of the volumeoccupancy less than 50%, a resistant value of silver layer 20 increases,and in the case of the volume occupancy exceeding 90%, bonding strengthbetween silver layer 20 and solid electrolyte layer 16 becomes weak.

The solid electrolytic capacitor of the comparison sample group findsbetter ESR characteristics at the volume occupancy ranging from 80 to90%; however, they are still poorer than those of sample groups 1–3.

The Phenolic novolak type epoxy resin represented by formula (1) and thetrishydroxyphenylmethane type epoxy resin represented by formula (2)have a plurality of reacting groups while conventional epoxy resin has asingle group. Therefore, they produce greater stress at their curing, sothat a contact pressure between the resin and the flaky silver particlesincreases. The resistant value of silver layer 20 is thus reduced, andadherence of silver layer 20 to solid electrolyte layer 16 or othersections of cathode layer 17 improves.

Silver paste formed of the mixture of flaky silver particles andspherical silver particles is prepared as sample group 4. The ratios offlaky silver particles vs. spherical silver particles in the mixture are80:20, 85:15, 90:10, 95:5, and 100:0. Other materials remain unchangedfrom those of sample group 1, and solid electrolytic capacitors ofsample group 4 are thus produced. The case where mixture ratio is 100:0is as same as one of sample group 1. Volume occupancy of silverparticles in silver layer 20 is set to 80%. An average particle diameterof the spherical silver particles is 4 μm.

FIG. 3 shows ESR characteristics measured at 100 kHz of the solidelectrolytic capacitor immediately after produced in accordance withsample group 4. As FIG. 3 tells, when an amount of flaky silverparticles is at least 90 wt. %, the ESR characteristics of thiscapacitor lowers to not more than 100 mΩ. When an amount of the flakysilver particles is less than 90 wt. %, a resistant value of silverlayer 20 increases, so that the ESR becomes high.

The foregoing discussion concludes that a solid electrolytic capacitorexcellent in ESR characteristics and impedance characteristics can beobtained when the following conditions are satisfied: silver layer 20includes the epoxy resins represented by formulae (1) and (2); an amountof flaky silver particles is not less than 90 wt. % in silver particles;and a volume occupancy of the flaky silver particles falls within50–90%.

The flaky silver particles of which diameters disperse from 0.1 to 30 μmand of which average particle diameter is 4 μm are used; however, theparticles of which average particle diameter ranges from 1–10 μm alsoproduce an advantage similar to what is discussed as above. It is morepreferable to use particles of which average diameter ranges from 3 to 8μm. If a particle diameter of the flaky silver particles in silver layer20 falls outside the range of 0.1–30 μm, silver particles agglomerate,or poorly contact with each other, so that a resistant value of silverlayer 20 increases.

It is preferable that the flaky shape of silver particles should bethis: the longitudinal length of a flat section ranges from two to tentimes of the thickness thereof. Use of the flaky silver particlesfalling within this range further lowers the resistant value of silverlayer 20. If the longitudinal length is less than two times of thethickness, silver particles contact with each other at point-to-point,so that the resistant value of silver layer 20 increases. If thelongitudinal length exceeds ten times of the thickness, the mixture inthe silver paste becomes uneven and the bonding strength of silver layer20 weakens. The resistant value of layer 20 as a whole thus increases.

In the present embodiment, cathode layer 17 is formed of silver layer 20and carbon layer 19. It is preferable for carbon layer 19 to use carbonsof which particle diameter is not greater than 5 μm. Cathode layer 17can be formed of only silver layer 20.

Second Exemplary Embodiment

In the second embodiment, the silver paste, of which flaky silverparticles occupy 80 vol. % of silver layer 20, is used. Heat treatmenttemperature after forming silver layer 20 is set between 160–250° C. insteps of 10° C. Other conditions remain unchanged from those of samplegroup 1 in the first embodiment, and solid electrolytic capacitors ofsample group 5 are thus produced. The capacitor having undergone theheat treatment at 200° C. is equal to the one of sample group 1.

FIG. 4 shows ESR characteristics measured at 100 kHz immediately afterthe production of the solid electrolytic capacitors in accordance withthe present embodiment and measured at 100 kHz of the same after 500hours at 105° C. with no load. FIG. 5 shows the measurement of curingstress of the silver paste. The curing stress means contraction stressproduced in curing of the silver paste applied and having undergone theheat treatment. The curing stress can be found by this method: thesilver paste is applied onto cover glass having a given thickness, andthe glass undergoes heat treatment. Then warp of the cover glass ismeasured for calculating the curing stress.

As FIG. 4 tells, the capacitors having undergone the heat treatment at180–230° C. show low ESR characteristics, this is shown by both thecapacitors measured immediately after the production and left at a hightemperature with no load. The capacitors out of this temperature rangeshow an increase in ESR characteristics.

As FIG. 5 tells, the curing stress of the silver paste depends on heattreatment temperature. Heat treatment at lower than 180° C. makes curingstress lower than 50 kg/cm² and bonding strength between the flakysilver particles and epoxy resin weak, so that ESR characteristicsbecome poor. On the other hand, heat treatment over 230° C. makes curingstress higher than 300 kg/cm²; however, cracks occur in silver layer 20or silver layer 20 peels off when it is left at a high temperature, sothat ESR characteristics become high. Detailed studies have found thatexcellent ESR characteristics are obtainable when the curing stressfalls within the range from 50 kg/cm² to 300 kg/cm².

The foregoing discussion concludes that it is preferable to set a heattreatment temperature between 180–230° C. in order to lower a contactresistance by bonding flaky silver particles with epoxy resin and alsolower the resistant value of silver layer 20.

Third Exemplary Embodiment

Sample 6 of the third embodiment is described first. In sample 6,cathode section 14 is dipped into water solution of 5% soluble aniline,dried in air before it undergoes heat treatment at 200° C. for 5minutes. This preparation produces a conductive polymer layer (notshown) which is a part of a solid electrolyte layer. This polymer layercorresponds to the manganese oxide layer used in sample group 1 in thefirst embodiment. Then pyrrole monomer 0.2 mol/L and naphthalensulfonatederivative 0.1 mol/L are dissolved into solvent formed by mixing waterand propyl alcohol, thereby producing polymerizing liquid to be used forforming the solid electrolyte. In this polymerizing liquid, an electrodefor starting polymerization is placed close to the surface of cathodesection 14, and electrolytic polymerization is carried out at apolymerization voltage of 1.5V. Solid electrolyte layer 16 having anaverage surface roughness of 3.8 μm is thus formed. Other conditions andmaterials remain unchanged from those of the first embodiment, so that asolid electrolytic capacitor is produced in a way similar to that ofsample group 1.

Next, sample 7 is described hereinafter. In sample 7, instead of thepolymerizing liquid used in sample group 1, thiophene monomer 0.05 mol/Land naphthalensulfonate derivative 0.03 mol/L are dissolved into solventformed by mixing water and ethanol, thereby producing polymerizingliquid to be used for forming the solid electrolyte. In thispolymerizing liquid, an electrode for starting polymerization is placedclose to the surface of cathode section 14, and electrolyticpolymerization is carried out at a polymerization voltage of 1.5V Otherconditions and materials remain unchanged from those of the firstembodiment, so that a solid electrolytic capacitor is produced in a waysimilar to that of sample group 1. Sample 7 thus produced has solidelectrolyte layer 16 having an average surface roughness of 0.1 μm.

Next, sample 8 is described hereinafter. In sample 8, instead of thepolymerizing liquid used in sample 7, ethylene-dioxythiophene monomer0.1 mol/L and naphthalensulfonate derivative 0.05 mol/L are dissolvedinto solvent formed by mixing water and propyl alcohol, therebyproducing polymerizing liquid to be used for forming the solidelectrolyte. In this polymerizing liquid, an electrode for startingpolymerization is placed close to the surface of cathode section 14, andelectrolytic polymerization is carried out at a polymerization voltageof 4V. Other conditions and materials remain unchanged from those ofsample 7, so that a solid electrolytic capacitor is produced. Sample 8has solid electrolyte layer 16 of which average surface roughness is30.0 μm

Next, sample 9 is described hereinafter. In sample 9, solid electrolytelayer 16 having an average surface roughness of 2.1 μm is produced inthe following manner: cathode section 14 is dipped into water solutionof 30% manganese nitrate, then dried in air before it undergoes heattreatment at 300° C. for 10 minutes. This set of the treatment isrepeated 15 times, so that a manganese oxide layer is produced as solidelectrolyte layer 16. Other conditions and materials remain unchangedfrom those of sample group 1 used in the first embodiment, so that asolid electrolytic capacitor is produced.

Next, sample 10 is described hereinafter. In sample 10, solidelectrolyte layer 16 having an average surface roughness of 3.5 μm isproduced in the same manner as sample 6. In the production, cathodesection 14 is dipped into water solution of 5% soluble aniline, thendried in air. Cathode section 14 then undergoes heat treatment at 210°C. for 3 minutes, so that a conductive polymer layer (not shown), whichis a part of the solid electrolyte layer, is formed. In the electrolyticpolymerization following the formation of polymer layer, after-mentionedpolymerizing liquid to be used for forming the solid electrolyte isused. The polymerizing liquid is formed in this way: pyrrole monomer 0.1mol/L and naphthalensulfonate derivative 0.1 mol/L are dissolved intosolvent formed by mixing water and propyl alcohol. Other conditions andmaterials remain unchanged from those of sample 6, so that a solidelectrolytic capacitor is produced.

Next, sample 11 is described hereinafter. In sample 11, solidelectrolyte layer 16 having an average surface roughness of 32.5 μm isproduced in the following manner: First, a conductive polymer layer (notshown), which is a part of the solid electrolyte layer, is formed in thesame manner as sample 10 is formed. Other conditions and materialsremain unchanged from those of sample 8, so that a solid electrolyticcapacitor is produced.

Next, sample 12 is described hereinafter. In sample 12, solidelectrolyte layer 16 having an average surface roughness of 12.5 μm isproduced in the following manner: cathode section 14 is dipped intowater-alcohol solution, in which 1.0% of polyethylene dioxy thiophenepolystylene sulfonic acid including binder and 1.0% of sulfonatedpolyaniline are dissolved, then pulled up. Cathode section 14 thenundergoes a drying process at 150° C. for 5 minutes. As a result,polyethylene dioxy thiophene polystylene sulfonate layer is formed. Thencathode section 14 is dipped into ethylene dioxy thiopene (heterocyclicmonomer) solution, and pulled up. This solution is mixed solutionincluding 1(one) part of ethylene dioxy thiophene as heterocyclicmonomer, 2 parts of ferric iron p-toluenesulfonate as oxidizing agent,and 4 parts of n-butanol as polymerization solvent. Then cathode section14 is left at 85° C. for 60 minutes, so that solid electrolyte layer 16formed of polyethylene dioxy thiophene, conductive polymer via chemicalpolymerization, is formed. Other conditions and materials remainunchanged from those of sample group 1 used in the first embodiment. Asolid electrolytic capacitor is thus produced.

Samples 6–12 employ silver layer 20 in which flaky silver particlesoccupying 80% volume in silver paste is used. Table 1 shows structuresof the solid electrolyte layers in samples 6–12.

TABLE 1 Solid Electrolyte Layer Sample 6 polyaniline/polypyrrole Sample7 manganese oxides/polythiophene Sample 8 manganese oxides/polyethylenedioxy thiophene Sample 9 Manganese oxides Sample 10polyaniline/polypyrrole Sample 11 polyaniline/polyethylene dioxythiophene Sample 12 polyethylene dioxy thiophene polystylene sulfonate/polyethylene dioxy thiophene

Table 2 shows ESR characteristics of samples 6–12, measured at initialstages of each sample, and after life test at 105° C. for 500 hours withno load, both at 100 kHz.

TABLE 2 ESR characteristics (mΩ) Right after Life test at averagesurface Production high Temp. roughness (μm) Sample 6 35 52 3.8 Sample 748 66 0.1 Sample 8 47 69 30.0 Sample 9 53 73 2.1 Sample 10 49 71 3.5Sample 11 89 106 32.5 Sample 12 56 83 12.5

Table 2 tells that use of thiophene, aniline, furan, or theirderivatives, or manganese oxides other than pyrrole in solid electrolytelayer 16 produces ESR characteristics similar to those of the firstembodiment. Use of such materials in solid electrolyte layer 16 producessolid electrolytic capacitors excellent in ESR characteristics andimpedance characteristics.

Use of an average surface roughness ranging from not less than 0.1 μm tonot greater than 30 μm increases contact areas between solid electrolytelayer 16 and silver layer 20 and decreases interface resistance, so thatESR characteristics and impedance characteristics improve.

In the foregoing embodiments, the valve metal of aluminum is used as ananode in the solid electrolytic capacitors; however, other valve metalssuch as tantalum, niobium, or titanium coated by oxide film can be usedwith an advantage similar to what is discussed above.

INDUSTRIAL APPLICABILITY

In the solid electrolytic capacitor of the present invention, adielectric oxide film layer, a solid electrolyte layer, and a cathodelayer are formed in this order on the surface of a valve metal. A partof the cathode layer is formed of a silver layer which includes silverparticles, and at least one of phenolic novolak type epoxy resinrepresented by formula (1), and a trishydroxyphenylmethane type epoxyresin represented by formula (2). Not less than 90 wt. % of the silverparticles are occupied by flaky silver particles, and a volume occupancyof the flaky silver particles falls within 50–90 vol. %. Phenolicnovolak type epoxy resin represented by formula (1) andtrishydroxyphenylmethane type epoxy resin represented by formula (2) aremore excellent in shrink characteristics in curing than that ofconventional epoxy resin. The contact pressure between the resin and theflaky silver particles thus increases, and a resistant value of thesilver layer decreases. Because the foregoing resins have pluralreacting groups, adherence to the solid electrolyte layer or the otherpart of the cathode layer improves. As a result, solid electrolyticcapacitors excellent in ESR characteristics and impedancecharacteristics are obtainable. As discussed above, the presentinvention relates to solid electrolytic capacitors and a process forproducing the same capacitors, and allows achieving a solid electrolyticcapacitor which has a greater capacity in downsized body in accordancewith a request from the market, and lowering its impedance in ahigh-frequency region.

1. A solid electrolytic capacitor comprising: a valve metal; adielectric oxide film layer disposed on a surface of the valve metal; asolid electrolyte layer disposed on a surface of the dielectric oxidefilm layer; and a cathode layer disposed on a surface of the solidelectrolytic capacitor and having a silver layer which includes silverparticles, and at least one of phenolic novolak type epoxy resinrepresented by formula (1) and trishydroxyphenylmethane type epoxy resinrepresented by formula (2), and not less than 90 wt. % of the silverparticles being occupied by flaky silver particles, and the flaky silverparticles occupying not less than 50 vol. % and not greater than 90 vol.% of the silver layer.


2. The solid electrolytic capacitor of claim 1, wherein a particlediameter of the flaky silver particles is not smaller than 0.1 μm andnot greater than 30 μm, and a longitudinal length of a flat section ofone of the flaky particles is not shorter than 2 times and not longerthan 10 times of a thickness of the flat section of one of the flakysilver particles.
 3. The solid electrolytic capacitor of claim 1,wherein the solid electrolyte layer has an average surface roughness ofnot smaller than 0.1 μm and not greater than 30 μm.
 4. The solidelectrolytic capacitor of claim 1, wherein the solid electrolyte layerincludes at least one of manganese oxide and conductive polymer of whichbasic skeleton is one of pyrrole, thiophene, aniline, furan, andderivatives thereof.
 5. A process for producing a solid electrolyticcapacitor, the process comprising the steps of: forming a dielectricoxide film layer on a surface of a valve metal; forming a solidelectrolyte layer on a surface of the dielectric oxide film layer; andforming a cathode layer, which includes a silver layer, on a surface ofthe solid electrolyte layer, wherein the silver layer is formed withsilver paste, the silver paste includes silver particles and at leastone of phenolic novolak type epoxy resin represented by formula (1) andtrishydroxyphenylmethane type epoxy resin represented by formula (2),and not less than 90 wt. % of the silver particles are occupied by flakysilver particles, which accounts for not less than 50 vol. % and notgreater than 90 vol. % in the silver layer.


6. The process of claim 5, wherein a particle diameter of the flakysilver particles is not less than 0.1 μm and not greater than 30 μm, anda longitudinal length of a flat section of one of the flaky particles isnot shorter than 2 times and not longer than 10 times of a thickness ofthe flat section of one of the flaky silver particles.
 7. The process ofclaim 5, wherein in the step of forming the cathode layer, the silverpaste undergoes heat treatment at a temperature of not lower than 180°C., and not higher than 230° C., and a curing stress of the silver layeris not smaller 50 kg/cm² and not greater than 300 kg/cm².